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Abstract:

A backlight unit that realizes stabilization of lighting operation in the
event of carrying out lighting operation at low luminance. The backlight
unit is backlight unit (10) comprising a light source composed of
multiple light emitting diode groups (4a, 4b, 31a, 31b, 31c, 31d) each
including a given number of serially connected light emitting diodes
(4,31); and lighting circuit (20, 40) for lighting the light source in
accordance with a light control signal. The lighting circuit has at least
multiple drive circuit portions (21a, 21b, 41a, 41b, 41c, 41d)
respectively connected to the multiple light emitting diode groups, and
in accordance with a light control signal, allows all the light emitting
diodes included in each of the multiple light emitting diode groups to
emit light, or allows only light emitting diodes included in light
emitting diode group(s) selected from among the multiple light emitting
diode groups to emit light.

Claims:

1. A backlight unit, comprising:a light source formed with a plurality of
light emitting-diode groups each including a predetermined number of
serially-connected light emitting diodes; anda lighting circuit that is
connected to the light source to light the light source according to a
light control signal fed from outside,whereinthe lighting circuit at
least has a plurality of drive circuit portions individually connected to
the plurality of light emitting-diode groups, andthe lighting circuit is
so configured as to make, according to the light control signal, all
light emitting diodes included in the plurality of light emitting-diode
groups or only light emitting diodes included in a predetermined light
emitting-diode group of the plurality of light emitting-diode groups emit
light.

2. The backlight unit of claim 1, wherein the plurality of light
emitting-diode groups are configured to be driven independently of one
another.

3. The backlight unit of claim 1,whereinthe lighting circuit further has a
comparator portion comparing the light control signal with a preset
threshold, andthe lighting circuit is so configured as to make, according
to a result of comparison between the light control signal and the
threshold, all light emitting diodes included in the plurality of light
emitting-diode groups or only light emitting diodes included in a
predetermined light emitting-diode group of the plurality of light
emitting-diode groups emit light.

4. The backlight unit of claim 1, wherein light emitting diodes included
in one light emitting-diode group of the plurality of light
emitting-diode groups are so configured as to continuously emit light
regardless of the light control signal.

5. The backlight unit of claim 4,whereinthe light emitting diodes are
classified into a low-brightness light emitting diode and a
high-brightness light emitting diode having emission brightness higher
than emission brightness of the low-brightness light emitting diode,
andthe light emitting diodes that are included in the one light
emitting-diode group of the plurality of light emitting-diode groups and
that are so configured as to continuously emit light are the
low-brightness light emitting diode.

6. The backlight unit of claim 1, wherein the plurality of light
emitting-diode groups are arranged such that light emitting diodes
included in the plurality of light emitting-diode groups are arranged by
turns in a predetermined direction.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a backlight unit, and in
particular, to a backlight unit using a plurality of light emitting
diodes as a light source.

BACKGROUND ART

[0002]LED (light emitting diode) drive units for driving a plurality of
serially-connected LEDs have conventionally been known (see, for example,
Patent Document 1). Backlight units provided with, for example, such LED
drive units have also been conventionally known.

[0003]FIG. 9 is a plan view schematically showing an example of the
structure of a conventional backlight unit. FIG. 10 is a diagram for
illustrating the structure of a light source of the conventional
backlight unit shown in FIG. 9. FIG. 11 is a diagram for illustrating the
structure of a lighting circuit connected to the light source of the
conventional backlight unit shown in FIG. 9. First, with reference to
FIGS. 9 to 11, a description will be given of an example of the structure
of a conventional backlight unit.

[0004]In the conventional backlight unit 110, as shown in FIG. 9, a light
guide plate 101, a light source (a plurality of LEDs 102), and the like
are accommodated inside a backlight frame 103. Although not illustrated,
a reflection sheet and an optical sheet are also accommodated inside the
backlight frame 103.

[0005]The plurality of LEDs 102 forming the light source are mounted on a
given substrate 104. Furthermore, the plurality of LEDs 102 are arranged
at predetermined intervals in a direction (direction A) along a light
entrance surface 101a of the light guide plate 101 such that each of the
LEDs 102 faces the light entrance surface 101a of the light guide plate
101.

[0006]As shown in FIG. 10, as well as being serially connected, the
plurality of LEDs 102 are structured such that they emit light by being
fed with a drive current from the lighting circuit 120. The lighting
circuit 120 feeding the plurality of LEDs 102 with a drive current, as
shown in FIG. 11, has one constant current circuit portion 121 and one
light control portion 122. The constant current circuit portion 121 has a
function of generating a predetermined constant current. The light
control portion 122 has a function of regulating the amount of drive
current fed to the plurality of LEDs 102 (see FIG. 10) according to a
light control signal fed from outside.

[0007]Lower-brightness lighting operation can be performed with the
above-described conventional backlight unit 110 than with a conventional
backlight unit using a CCFT (cold cathode fluorescent tube) as the light
source. Incidentally, the LEDs 102 emit light with a current equal to or
less than a several percent (for example, approximately 1%) of a rated
current.

[0008]Patent Document 1: JP-A-2007-42758

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

[0009]With the above-described conventional backlight unit 110, when the
drive current fed to the LEDs 102 is decreased to a several percent (for
example, approximately 1%) of the rated current or less to perform
low-brightness lighting operation, the drive current fed from the
lighting circuit 120 inconveniently contains electrical noise, or the
waveform of the drive current fed from the lighting circuit 120 is
inconveniently deformed, which will invite the following problem. That
is, in performing low-brightness lighting operation with the
above-described conventional backlight unit 110, it is difficult for the
lighting circuit 120 to stably output a drive current which is several
percent of the rated current or less, and this results in unstable
lighting operation.

[0010]For example, assuming that the drive current fed to the LEDs 102 in
high-brightness lighting operation is approximately 150 mA (light control
signal: 100%) and that the drive current contains electrical noise of
approximately ±1 mA, the drive current actually fed to the LEDs 102 is
approximately 149 to 151 mA. In this case, the electrical noise can be
said to have hardly any negative effect on the lighting operation.

[0011]On the other hand, assuming that the drive current fed to the LEDs
102 in low-brightness lighting operation is approximately 1.5 mA (light
control signal: 1%), and that the drive current contains electrical noise
of approximately ±1 mA, the drive current actually fed to the LEDs 102
is approximately 0.5 to 2.5 mA. In this case, the drive current actually
fed to the LEDs 102 changes by up to approximately five times, and this
results in unstable lighting operation.

[0012]The present invention has been made to solve the above-described
problems, and an object of the invention is to provide a backlight unit
allowing stable low-brightness lighting operation.

Means for Solving the Problem

[0013]To achieve the above object, according to one aspect of the present
invention, a backlight unit is provided with: a light source formed with
a plurality of light emitting-diode groups each including a predetermined
number of serially-connected light emitting diodes; and a lighting
circuit that is connected to the light source to light the light source
according to a light control signal fed from outside. Here, the lighting
circuit at least has a plurality of drive circuit portions individually
connected to the plurality of light emitting-diode groups, and the
lighting circuit is so configured as to make, according to the light
control signal, all light emitting diodes included in the plurality of
light emitting-diode groups or only light emitting diodes included in a
predetermined light emitting-diode group of the plurality of light
emitting-diode groups emit light.

[0014]With the backlight unit according to the one aspect of the present
invention described above, by using the lighting circuit that at least
has the plurality of drive circuit portions individually connected to the
plurality of light emitting-diode groups, the plurality of light
emitting-diode groups can be driven independently of one another. This
makes it possible to make all the light emitting diodes included in the
plurality of light emitting-diode groups emit light, and to make only
light emitting diodes included in a predetermined one of the plurality of
light emitting-diode groups emit light. This makes it possible to
stabilize lighting operation when lighting operation is performed at low
brightness. A specific description will be given below of advantages of
the backlight unit according to the one aspect of the present invention.
Incidentally, in the following description, it is assumed that two light
emitting-diode groups are provided. In the following description, it is
also assumed that a maximum-brightness state is a 100%-brightness state
and that lighting operation can be stably performed if the light control
signal is approximately 1.4% or more.

[0015]That is, if, in the case in which the light control signal is
approximately 1.4%, only the light emitting diodes included in one of the
two light emitting-diode groups are made to emit light, then only half of
all the light emitting diodes forming the light source emit light and the
other half of the light emitting diodes stop emitting light. Thus, in the
case in which the light control signal is approximately 1.4%, it is
possible to make light amount of the light source approximately 0.7%.
Thus, low-brightness lighting operation can be stably performed until the
light amount of the light source reaches approximately 0.7%.
Incidentally, with the conventional configuration shown in FIGS. 9 to 11,
since it is impossible to make only predetermined light emitting diodes
of all the light emitting diodes forming the light source emit light, if
the light control signal is approximately 1.4%, the light amount of the
light source is also approximately 1.4%. And, if the light amount of the
light source is reduced to approximately 0.7%, it is difficult to perform
stable lighting operation.

[0016]In the backlight unit according to the above-described one aspect of
the present invention, it is preferable that the plurality of light
emitting-diode groups be configured to be driven independently of one
another. This configuration makes it easy to make all the light emitting
diodes included in the plurality of light emitting-diode groups emit
light and to make only the light emitting diodes included in a
predetermined light emitting-diode group of the plurality of light
emitting-diode groups emit light.

[0017]In the backlight unit according to the above-described one aspect of
the present invention, it is preferable that the lighting circuit further
have a comparator portion comparing the light control signal with a
preset threshold, and that the lighting circuit be so configured as to
make, according to a result of comparison between the light control
signal and the threshold, all light emitting diodes included in the
plurality of light emitting-diode groups or only light emitting diodes
included in a predetermined light emitting-diode group of the plurality
of light emitting-diode groups emit light. This configuration makes it
easy to judge whether to make all the light emitting diodes included in
the plurality of light emitting-diode groups to emit light or to make
only the light emitting diodes included in a predetermined light
emitting-diode group of the plurality of light emitting-diode groups to
emit light.

[0018]In the backlight unit according to the above-described one aspect of
the present invention, it is preferable that light emitting diodes
included in one light emitting-diode group of the plurality of light
emitting-diode groups be so configured as to continuously emit light
regardless of the light control signal

[0019]In this case, it is preferable that the light emitting diodes be
classified into a low-brightness light emitting diode and a
high-brightness light emitting diode having emission brightness higher
than emission brightness of the low-brightness light emitting diode, and
that the light emitting diodes that are included in the one light
emitting-diode group of the plurality of light emitting-diode groups and
that are so configured as to continuously emit light be each the
low-brightness light emitting diode. With this configuration, in
performing lighting operation at low brightness, the light amount of the
light source can be further reduced by making only the light emitting
diodes (low-brightness light emitting diodes configured to continuously
emit light) included in the light emitting-diode group of the plurality
of light emitting-diode groups emit light.

[0020]In the backlight unit according to the above-described one aspect of
the present invention, it is preferable that the plurality of light
emitting-diode groups be arranged such that light emitting diodes
included in the plurality of light emitting-diode groups are arranged by
turns in a predetermined direction. With this configuration, in a case in
which only light emitting diodes included in a predetermined light
emitting-diode group of the plurality of light emitting-diode groups are
made to emit light, it is possible to prevent it from happening that
light emitting diodes emitting light are concentratedly disposed in a
certain location. This helps prevent non-uniformity from occurring in
brightness.

ADVANTAGES OF THE INVENTION

[0021]As described above, according to the present invention, it is
possible to easily obtain a backlight unit allowing lighting operation to
be stabilized in a case in which the lighting operation is performed at
low brightness.

BRIEF DESCRIPTION OF DRAWINGS

[0022]FIG. 1 is an exploded perspective view showing the structure of a
liquid crystal display device including a backlight unit according to a
first embodiment of the present invention;

[0023]FIG. 2 is a sectional view showing the backlight unit according to
the first embodiment shown in FIG. 1;

[0024]FIG. 3 is a diagram for illustrating the structure of a light source
of the backlight unit according to the first embodiment shown in FIG. 1;

[0025]FIG. 4 is a diagram for illustrating the structure of a lighting
circuit connected to the light source of the backlight unit according to
the first embodiment shown in FIG. 1;

[0026]FIG. 5 is a diagram for illustrating lighting operation of the
backlight unit according to the first embodiment of the present
invention;

[0027]FIG. 6 is a diagram for illustrating the structure of a light source
of a backlight unit according to a second embodiment of the present
invention;

[0028]FIG. 7 is a diagram for illustrating the structure of a lighting
circuit connected to the light source of the backlight unit according to
the second embodiment shown in FIG. 6;

[0029]FIG. 8 is a diagram for illustrating lighting operation of the
backlight unit according to the second embodiment of the present
invention;

[0030]FIG. 9 is a plan view schematically showing an example of the
structure of a conventional backlight unit;

[0031]FIG. 10 is a diagram for illustrating the structure of a light
source of the conventional backlight unit shown in FIG. 9; and

[0032]FIG. 11 is a diagram for illustrating the structure of a lighting
circuit connected to the light source of the conventional backlight unit
shown in FIG. 9.

[0038]First, with reference to FIGS. 1 to 4, descriptions will be given of
the structure of a backlight unit according to a first embodiment and
that of a liquid crystal display device including the backlight unit.

[0039]As shown in FIG. 1, the liquid crystal display device including the
backlight unit 10 of the first embodiment is used in a state in which the
backlight unit 10 is disposed on a rear side of a liquid crystal display
panel 50. And the backlight unit 10 of the first embodiment is designed
to emit surface light toward the liquid crystal display panel 50 from the
rear side of the liquid crystal display panel 50. A detailed description
will be given below of the structure of the backlight unit 10 of the
first embodiment.

[0040]As shown in FIGS. 1 and 2, the backlight unit 10 of the first
embodiment is provided at least with a backlight frame 1, a reflection
sheet 2, a light guide plate 3, a light source formed with a plurality of
LEDs (light emitting diodes) 4, and a plurality of optical sheets 5.

[0041]The backlight frame 1 is made of a white plastic or the like, and
has four side portions 1a to 1d connected in a frame shape and a bottom
portion 1e disposed behind the frame body composed of the four side
portions 1a to 1d. And members (the reflection sheet 2, the light guide
plate 3, the LEDs 4, and the optical sheets 5) forming the
above-described backlight unit 10 are held in a region (an accommodation
region) surrounded by the four side portions 1a to 1d of the backlight
frame 1. Specifically, the reflection sheet 2, the light guide plate 3,
and the optical sheets 5 are sequentially placed one on another in this
order on the bottom portion 1e of the backlight frame 1. The LEDs 4 are
fitted to the side portion 1a of the backlight frame 1.

[0042]The reflection sheet 2 is a formed with, for example, a resin sheet
member capable of reflecting light, and so disposed as to cover a rear
surface 3f of the light guide plate 3, which will be described later.
With the provision of the reflection sheet 2, even if light is emitted
toward a rear side from the rear surface 3f of the light guide plate 3,
the light is reflected on the reflection sheet 2. This makes it possible
for the light emitted toward the rear side from the rear surface 3f of
the light guide plate 3 to be reintroduced into the light guide plate 3.

[0043]The light guide plate 3 is formed with a transparent member formed
with a transparent resin, and has at least four side surfaces 3a to 3d.
In a state in which the light guide plate 3 is accommodated inside the
backlight frame 1, the four side surfaces 3a to 3d of the light guide
plate 3 are arranged along the four side portions 1a to 1d, respectively,
of the backlight frame 1. And the side surface 3a of the light guide
plate 3 disposed along the side portion 1a of the backlight frame 1
functions as a light entrance surface through which light generated in
the LEDs 4 is introduced into the light guide plate 3. In the following
description, the side surface 3a will be referred to as a light entrance
surface 3a.

[0044]In addition, the light guide plate 3 has one front surface 3e and
one rear surface 3f as well. The front surface 3e of the light guide
plate 3 functions as a light emission surface through which light
introduced into the light guide plate 3 is emitted towards a front side
(liquid crystal display panel 50 side). In the following description, the
side surface 3e will be referred to as a light emission surface 3e. The
provision of the light guide plate 3 makes it possible to make light
generated in the LEDs 4 travel toward the front side (liquid crystal
display panel 50 side).

[0045]The plurality of LEDs 4 forming the light source are arranged in one
line at predetermined intervals in a direction (direction A) along the
light entrance surface 3a of the light guide plate 3 such that each of
the LEDs 4 faces the light entrance surface 3a of the light guide plate
3. The plurality of LEDs 4 are mounted on a same LED substrate 6. And the
LED substrate 6 on which the plurality of LEDs 4 are mounted is adhered
to the side portion 1a of the backlight frame 1 with a double-faced tape
(not shown) or the like.

[0046]The plurality of optical sheets 5 include a diffusion sheet, a prism
sheet, and the like. The plurality of optical sheets 5 perform diffusion
and the like of light emitted from the light emission surface 3e of the
light guide plate 3.

[0047]Incidentally, although not illustrated, a member having a function
of, for example, pressing the optical sheets 5 from a front side is
provided on the front side of the optical sheets 5. This member may be
integrally formed with the backlight frame 1, or may be fitted to the
backlight frame 1 after the backlight frame 1 is formed.

[0048]Here, in the first embodiment, as shown in FIG. 3, the plurality of
LEDs 4 forming the light source are classified into two LED groups (light
emitting-diode groups) 4a and 4b. These two LED groups 4a and 4b each
include a predetermined number of serially-connected LEDs 4. And the two
LED groups 4a and 4b are arranged such that the LEDs 4 included in the
two LED groups 4a and 4b are alternately arranged in the direction A.
That is, the LEDs 4 of the LED group 4a and those of the LED group 4b
appear to be arranged in a nested state.

[0049]In addition, although the plurality of LEDs 4 of the first
embodiment are of a type, they are not uniform in brightness of emission
light. That is, the plurality of LEDs 4 can be classified into
low-brightness LEDs 4 and high-brightness LEDs 4 having emission
brightness that is higher than emission brightness of the low brightness
LEDs 4. And, in the first embodiment, the LED group 4a is composed of a
predetermined number of low-brightness LEDs 4 and the LED group 4b is
composed of a predetermined number of high-brightness LEDs 4.

[0050]In addition, in the first embodiment, the two LED groups 4a and 4b
are driven independently of each other. Specifically, the two LED groups
4a and 4b are connected to a lighting circuit 20 that is capable of
driving the two LED groups 4a and 4b independently of each other. The
lighting circuit 20, as shown in FIGS. 3 and 4, at least has two drive
circuit portions 21a and 21b that are individually connected to the two
LED groups 4a and 4b, respectively. And the LED group 4a is driven by the
drive circuit portion 21a separately from the LED group 4b, and the LED
group 4b is driven by the drive circuit portion 21b separately from the
LED group 4a.

[0051]The drive circuit portion 21a includes a constant current circuit
portion 22a and a light control portion 23a, and the drive circuit
portion 21b includes a constant current circuit portion 22b and a light
control portion 23b. The constant current circuit portion 22a has a
function of generating a predetermined constant current. The light
control portion 23a has a function of regulating the amount of drive
current fed to the LED group 4a (regulating the light amount of the LEDs
4 included in the LED group 4a) according to a light control signal fed
from outside. Incidentally, the constant current circuit portion 22b has
the same function as the constant current circuit portion 22a described
above, and the light control portion 23b has the same function as the
light control portion 23a described above.

[0052]The lighting circuit 20 of the first embodiment further has, in
addition to the drive circuit portions 21a and 21b, a comparator portion
24 and a threshold setting portion 25. The comparator portion 24 has a
function of comparing a light control signal fed from outside with a
preset threshold and feeding an enable signal to the light control
portion 23b connected to the light emitting-diode group 4b. The threshold
setting portion 25 has a function of feeding a threshold signal to the
comparator portion 24.

[0053]Now, the comparator portion 24 is so configured as to generate a
high-level enable signal or a low-level enable signal according to a
result obtained by comparing a light control signal with the threshold.
Specifically, the comparator portion 24 generates a high-level enable
signal if the light control signal is equal to or higher than the
threshold, and generates a low-level enable signal if the light control
signal is lower than the threshold. Furthermore, the light control
portion 23b, to which an enable signal is fed, is so configured as to
feed a drive current to the LED group 4b when the enable signal is high
level, and stop feeding the drive current to the LED group 4b when the
enable signal is low level. That is, in the first embodiment, in a case
in which it is judged at the comparator portion 24 that the light control
signal is equal to or higher than the threshold, the LEDs 4 included in
the LED group 4b emit light, and in a case in which it is judged at the
comparator portion 24 that the light control signal is lower than the
threshold, the LEDs 4 included in the LED group 4b stop emitting light.

[0054]The enable signal generated at the comparator portion 24 is not fed
to the light control portion 23a connected to the LED group 4a. Thus, the
LED group 4a is constantly supplied with a drive current from the light
control portion 23a regardless of the light control signal. In other
words, the LEDs 4 included in the LED group 4a continuously emit light
regardless of the light control signal. Incidentally, the LEDs 4 included
in the LED group 4a are low-brightness LEDs 4.

[0055]With the backlight unit 10 as described above, it is possible to
make, according to the light control signal fed from outside, all the
LEDs 4 included in the two LED groups 4a and 4b or only the LEDs 4
included in the LED group 4a emit light. Specifically, in a case in which
the light control signal is equal to or higher than the threshold, all
the LEDs 4 included in the two LED groups 4a and 4b emit light, and in a
case in which the light control signal is lower than the threshold, only
the LEDs 4 included in the LED group 4a emit light.

[0056]Next, with reference to FIGS. 3 to 5, a description will be given of
lighting operation of the backlight unit according to the first
embodiment. In FIG. 5, "LED 1" denotes an LED 4 included in the LED group
4a, and "LED 2" denotes an LED 4 included in the LED group 4b. In FIG. 5,
an LED 4 that is not emitting light is illustrated with hatching, while
an LED 4 that is emitting light is illustrated without hatching. In the
following description, it is assumed that a maximum-brightness state is a
100%-brightness state and the threshold is 10%.

[0057]First, the LED group 4a is constantly supplied with a drive current
from the light control portion 23a regardless of the light control
signal. Thus, the LEDs 4 included in the LED group 4a continuously emit
light regardless of the light control signal.

[0058]And, in a case in which the light control signal is between
approximately 100% and approximately 10%, the comparator portion 24
judges that the light control signal is equal to or higher than the
threshold. Thus, the comparator portion 24 generates a high-level enable
signal, and the high-level enable signal is fed to the light control
portion 23b. As a result, a drive current is supplied from the light
control portion 23b to the LED group 4b, and thus the LEDs 4 included in
the LED group 4b emit light.

[0059]Thus, in the case in which the light control signal is between
approximately 100% and approximately 10%, among the plurality of LEDs 4
forming the light source, all the LEDs 4 included in the two LED groups
4a and 4b emit light. As a result, in the case in which the light control
signal is between approximately 100% and approximately 10%, the light
amount of the light source composed of the plurality of LEDs 4 is between
approximately 100% and approximately 10%.

[0060]Also, in a case in which the light control signal is equal to or
higher than approximately 1% but lower than approximately 10%, it is
judged at the comparator portion 24 that the light control signal is
lower than the threshold. Thus, the comparator portion 24 generates a
low-level enable signal, and the low-level enable signal is fed to the
light control portion 23b. As a result, supply of a drive current from
the light control portion 23b to the LED group 4b is stopped, and thus
the LEDs 4 included in the LED group 4b stop emitting light.

[0061]Thus, in the case in which the light control signal is equal to or
higher than approximately 1% but lower than approximately 10%, among the
plurality of LEDs 4 forming the light source, only the LEDs 4 included in
the LED group 4a emit light. In other words, only half of the plurality
of LEDs 4 forming the light source emit light. As a result, in the case
in which the light control signal is equal to or higher than
approximately 1% but lower than approximately 10%, the light amount of
the light source composed of the plurality of LEDs 4 is equal to or
higher than approximately 0.5% but lower than approximately 5%.

[0062]In the first embodiment, as described above, by using the lighting
circuit 20 at least having the drive circuit portions 21a and 21b that
are separately connected to the LED groups 4a and 4b, respectively, it is
possible to drive the two LED groups 4a and 4b independently of each
other. This makes it possible to make all the LEDs 4 included in the two
LED groups 4a and 4b emit light and to make only the LEDs 4 included in
the LED group 4a of the two LED groups 4a and 4b emit light. For example,
assuming that the light control signal is approximately 1.4%, only one
half of all the LEDs 4 forming the light source emit light, and the other
half of the LEDs stop emitting light. That is, in a case in which the
light control signal is approximately 1.4%, the light amount of the light
source is approximately 0.7%. In this case, assuming that lighting
operation can be stably performed if the control signal is equal to
approximately 1.4% or higher, low-brightness lighting operation can be
stably performed until the light amount of the light source reaches
approximately 0.7%.

[0063]Also, in the first embodiment, as described above, the provision of
the comparator portion 24 comparing the light control signal with the
threshold makes it easy to judge whether to make all the LEDs 4 included
in the two LED groups 4a and 4b emit light or to make only the LEDs 4
included in the LED group 4a of the two LED groups 4a and 4b emit light.

[0064]Also, in the first embodiment, as described above, by building the
LEDs 4 that are included in the LED group 4a and that continuously emit
light as low-brightness LEDs 4, it is possible to further reduce the
light amount of the light source in low-brightness lighting operation
performed by making only the LEDs 4 included in the LED group 4a emit
light.

[0065]Also, in the first embodiment, as described above, by alternately
arranging the LEDs 4 of the LED group 4a and the LEDs 4 of the LED group
4b in the direction (direction A) along the light entrance surface 3a of
the light guide plate 3, it is possible to prevent it from happening that
light emitting diodes emitting light are concentratedly disposed in a
certain location. This helps prevent non-uniformity from occurring in
brightness.

Second Embodiment

[0066]Next, with reference to FIGS. 6 and 7, a description will be given
of the structure of a backlight unit according to a second embodiment.

[0067]As shown in FIG. 6, a light source of the backlight unit of the
second embodiment is composed of a plurality of LEDs (light emitting
diodes) 31. The plurality of LEDs 31 are, although not illustrated,
arranged in one line at predetermined intervals in a direction A (a
direction along a light entrance surface of a light guide plate) such
that each of the LEDs 31 faces the light entrance surface of the light
guide plate. The plurality of LEDs 31 are mounted on a same LED substrate
32.

[0068]Here, in the second embodiment, the plurality of LEDs 31 are
classified into four LED groups 31a to 31d. These four LED groups 31a to
31d each include a predetermined number of serially-connected LEDs 31.
And the four LED groups 31a to 31d are arranged such that the LEDs 31
included in each of the four LED groups 31a to 31d are alternately
arranged in the direction A. Incidentally, for the purpose of
simplification of the drawing, conductors for serially connecting a
predetermined number of LEDs 31 are not illustrated in FIG. 6.

[0069]In the second embodiment, the four LED groups 31a to 31d are
connected to a lighting circuit 40 that can drive the four LED groups 31a
to 31d independently of one another. As shown in FIGS. 6 and 7, the
lighting circuit 40 at least has four drive circuit portions 41a to 41d
connected to the four LED groups 31a to 31d, respectively. And the LED
group 31a is driven by the drive circuit portion 41a separately from the
LED groups 31b to 31d, and the LED group 31b is driven by the drive
circuit portion 41b separately from the LED groups 31a, 31c, and 31d. The
LED group 31c is driven by the drive circuit portion 41c separately from
the LED groups 31a, 31b, and 31d, and the LED group 31d is driven by the
drive circuit portion 41d separately from the LED groups 31a to 31c.

[0070]The drive circuit portions 41a to 41d include constant current
circuit portions 42a to 42d, respectively, and light control portions 43a
to 43d, respectively. The constant current circuit portion 42a has a
function of generating a predetermined constant current. The light
control portion 43a has a function of regulating the amount of drive
current fed to the LED group 31a (regulate light amount of the LEDs 31
included in the LED group 31a) according to a light control signal fed
from outside. Incidentally, the constant current circuit portions 42b to
42d each have the same function as the constant current circuit portions
42a described above, and the light control portions 43b to 43d each have
the same function as the light control portion 43a described above.

[0071]The lighting circuit 40 of the second embodiment also has a
comparator portion 44 and a threshold setting portion 45. The comparator
portion 44 has a function of comparing the light control signal fed from
outside with a preset threshold and feeding an enable signal to the light
control portions 43b to 43d connected to the light emitting-diode groups
31b to 31d. The threshold setting portion 45 has a function of feeding a
threshold signal to the comparator portion 44.

[0072]Now, the comparator portion 44 is so configured as to generate,
according to a result obtained by comparing the light control signal with
the threshold, both a high-level enable signal and a low-level enable
signal or only a high-level enable signal or a low-level enable signal.
Furthermore, the light control portion 43b, to which the enable signal is
fed, is so configured as to feed a drive current to the LED group 31b
when the enable signal is high level, and to stop feeding the drive
current to the LED group 31b when the enable signal is low level. The
light control portions 43c and 43d each have the same function as the
light control portion 43b described above. Thus, with the second
embodiment, it is possible, according to the result obtained by comparing
the light control signal with the threshold, to make all the LEDs 31
included in the LED groups 31b to 31d emit light, to make only the LEDs
31 included in one of the LED groups 31b to 31d emit light, or to make
all the LEDs 31 included in the LED groups 31b to 31d stop emitting
light.

[0073]The enable signal generated by the comparator portion 44 is not fed
to the light control portion 43a connected to the LED group 31a. Thus,
the LED group 31a is constantly supplied with a drive current from the
light control portion 43 a regardless of the light control signal. In
other words, the LEDs 31 included in the LED group 31a continuously emit
light regardless of the light control signal.

[0074]The lighting circuit 40 of the second embodiment also has a filter
portion 46 and an amplifier portion 47. The filter portion 46 has a
function of removing noise from the light control signal to be fed to the
amplifier portion 47. The amplifier portion 47 has a function of
amplifying the light control signal according to the enable signal fed
from the comparator portion 44. The light control signal is fed to the
light control portions 43a to 43d after being subjected to the processing
by the filter portion 46 and by the amplifier portion 47.

[0075]In the other respects, the structure of the second embodiment is
similar to that of the above-described first embodiment.

[0076]Next, with reference to FIGS. 6 to 8, a description will be given of
lighting operation of a backlight unit according to the second
embodiment. In FIG. 8, "LED 1" denotes an LED 31 included in the LED
group 31a, and "LED 2" denotes an LED 31 included in the LED group 31b.
In FIG. 8, "LED 3" denotes an LED 31 included in the LED group 31c, and
"LED 4" denotes an LED 31 included in the LED group 31d. In FIG. 8, an
LED 31 that is not emitting light is illustrated with hatching, while an
LED 4 that is emitting light is illustrated without hatching. In the
following description of operation, it is assumed that a
maximum-brightness state is a 100%-brightness state and thresholds are
10%, 5%, and 1%.

[0077]First, the LED group 31a is constantly supplied with a drive current
from the light control portion 43a regardless of the light control
signal. Thus, the LEDs 31 included in the LED group 31a continuously emit
light regardless of the light control signal.

[0078]In a case in which the light control signal is between approximately
100% and approximately 10%, the comparator portion 44 generates only a
high-level enable signal, and the high-level enable signal is fed to the
light control portions 43b to 43d. As a result, a drive current is
supplied from the light control portions 43b to 43d to the LED groups 31b
to 31d, and thus the LEDs 31 included in the LED groups 31b to 31d emit
light.

[0079]Thus, in the case in which the light control signal is between
approximately 100% and approximately 10%, among the plurality of LEDs 31
forming the light source, all the LEDs 31 included in the LED groups 31a
to 31d emit light. In this case, amplification of the light control
signal is not performed at the amplifier 47.

[0080]In a case in which the light control signal is equal to or higher
than approximately 5% but lower than approximately 10%, the comparator
portion 44 generates both a high-level enable signal and a low-level
enable signal, to feed the high-level enable signal to the light control
portions 43b and 43c, and the low-level enable signal to the light
control portion 43d. Thus, since a drive current is supplied from the
light control portions 43b and 43c to the LED groups 31b and 31c, the
LEDs 31 included in the LED groups 31b and 31c emit light. On the other
hand, no drive current is supplied from the light control portion 43d to
the LED group 31d, and thus the LEDs 31 included in the LED group 31d do
not emit light.

[0081]As a result, in the case in which the light control signal is equal
to or higher than approximately 5% but lower than approximately 10%,
among the plurality of LEDs 31 forming the light source, only the LEDs 31
included in the LED groups 31a to 31c emit light. In other words, only
3/4 of the plurality of LEDs 31 forming the light source emit light, the
light amount of the light source composed of the plurality of LEDs 31 is
reduced to 3/4 times. Incidentally, in this case, the amplifier 47
amplifies the light control signal by a factor of 4/3.

[0082]In a case in which the light control signal is equal to or higher
than approximately 1% but lower than approximately 5%, the comparator
portion 44 generates both a high-level enable signal and a low-level
enable signal, to feed the high-level enable signal to the light control
portion 43c, and the low-level enable signal to the light control
portions 43b and 43d. Thus, since a drive current is supplied from the
light control portion 43c to the LED group 31c, the LEDs 31 included in
the LED group 31c emit light. On the other hand, no drive current is
supplied from the light control portions 43b and 43d to the LED groups
31b and 31d, and thus the LEDs 31 included in the LED groups 31b and 31d
do not emit light.

[0083]As a result, in the case in which the light control signal is equal
to or higher than approximately 1% but lower than approximately 5%, among
the plurality of LEDs 31 forming the light source, only those included in
the LED groups 31a and 31c emit light. In other words, only 2/4 of the
plurality of LEDs 31 forming the light source emit light, and thus the
light amount of the light source composed of the plurality of LEDs 31 is
reduced to 2/4 times. Incidentally, in this case, amplification of the
light control signal is performed at the amplifier 47 to amplify the
light control signal by a factor of 2.

[0084]In a case in which the light control signal is equal to or higher
than approximately 0.5% but lower than approximately 1%, the comparator
portion 44 generates only a low-level enable signal and the low-level
enable signal is fed to the light control portions 43b to 43d. Thus,
since no drive current is supplied from the light control portions 43b to
43d to the LED groups 31b to 31d, the LEDs 31 included in the LED groups
31b to 31d do not emit light.

[0085]As a result, in the case in which the light control signal is equal
to or higher than approximately 0.5% but lower than approximately 1%,
among the plurality of LEDs 31 forming the light source, only those
included in the LED group 31a emit light. In other words, only 1/4 of the
plurality of LEDs 31 forming the light source emit light, and thus the
light amount of the light source composed of the plurality of LEDs 31 is
reduced to 1/4 times. Incidentally, in this case, amplification of the
light control signal is performed at the amplifier 47 so as to amplify
the light control signal by a factor of 4.

[0086]In the other respects, the structure of the second embodiment is
similar to that of the above-described first embodiment.

[0087]The embodiments disclosed herein are to be considered in all
respects as illustrative and not restrictive. The scope of the present
invention is set out in the appended claims and not in the descriptions
of the embodiments hereinabove, and includes any variations and
modifications within the sense and scope equivalent to those of the
claims.

[0088]For example, in the above-described embodiments, the plurality of
LEDs are classified into two or four LED groups, but this is not meant to
limit the present invention, and the plurality of LEDs may be classified
into three LED groups, or into five or more LED groups.

[0089]Furthermore, the light control signal is not subjected to noise
removal processing or amplification in the above-described first
embodiment, but this is not meant to limit the present invention, and the
light control signal may be subjected to noise removal processing and
amplification as in the above-described second embodiment.

[0090]Moreover, the light control signal is subjected to noise removal
processing or amplification in the above-described second embodiment, but
this is not meant to limit the present invention, and the light control
signal may not be subjected to noise removal processing or amplification
as in the above-described first embodiment.